You also need to take into consideration those who find the stones, and want the most GC for it.. They wont sell it to someone for say, 8k.. when a bot is paying 13k for it.
I don't think the prices will go down anytime soon, unless the chance to find a stone is increased from what it is now, so there is more in-game..
Also with so many different items using the same 3-4 ingredients to make them (Serp Stones, Bindings, EFEs, EMEs) it's no wonder the prices are high, and they are harder to find.. There needs to be more items that can be used to create the items, more stones, or something.. A wider selection of items that can be used to create another item, the better it is for the prices of the items.
Anyways, good luck with trying to get the prices to drop.